1. Field of the Invention
The present invention generally relates to a mobile communication system. More particularly, the present invention relates to a method and apparatus for efficiently detecting contention, for reducing signaling overhead during a random access procedure.
2. Description of the Related Art
The Universal Mobile Telecommunications System (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in Wideband Code Division Multiple Access (WCDMA), based on European mobile communication systems, Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS). The 3rd Generation Partnership Project (3GPP) that standardized UMTS is now discussing Long Term Evolution (LTE) as the next generation of UMTS, the Evolved UMTS. The 3GPP LTE is a technology for enabling packet communications at or above 100 Mbps, aiming at commercialization by 2010. For deploying the LTE system, many communication schemes have been proposed. Among them, there are schemes of reducing the number of nodes on a communication line by simplifying a network configuration or of optimizing radio protocols for radio channels.
FIG. 1 is an exemplary view illustrating an Evolved 3GPP LTE system.
Referring to FIG. 1, each of Evolved UMTS Radio Access Networks (E-UTRANs or E-RANs) 110 and 112 is simplified to a 2-node structure including Evolved Node Bs (ENBs) 120, 122, 124 and an anchor node 130, or ENBs 126 and 128 and an anchor node 132. A User Equipment (UE) 101 is connected to an Internet Protocol (IP) network 114 via the E-RAN 110 or 112. The ENBs 120 to 128 correspond to legacy Node Bs in the UMTS system and are connected to the UE 101 via radio channels.
Compared to the legacy Node Bs, the ENBs 120 to 128 play a more complex role. Since all user traffic including real-time service such as Voice over IP (VoIP) is serviced on shared channels in the 3GPP LTE, an entity for collecting the status information of UEs and scheduling them is required and the ENBs 120 to 128 are responsible for the scheduling. Generally, an ENB controls a plurality of cells.
The ENBs 120 to 128 also perform Adaptive Modulation and Coding (AMC) by selecting a modulation scheme and a channel coding rate for a UE adaptively according to the channel status of the UE. As with High Speed Downlink Packet Access (HSDPA) of UMTS and High Speed Uplink Packet Access (HSUPA) (or Enhanced Dedicated CHannel (EDCH)), the LTE system uses Hybrid Automatic Repeat reQuest (HARQ) between the ENBs 120 to 128 and the UE 101. Considering that a variety of Quality of Service (QoS) requirements cannot be fulfilled with HARQ alone, a high layer may perform an outer ARQ between the UE 101 and the ENBs 120 to 128. HARQ is a technique for increasing reception success rate by soft-combining previous received data with retransmitted data without discarding the previous data. High-speed packet communication systems such as HSDPA and EDCH use HARQ to increase transmission efficiency. To realize a data rate of up to 100 Mbps, it is expected that the LTE system will adopt Orthogonal Frequency Division Multiplexing (OFDM) in a 20-MHz bandwidth as a radio access technology.
FIG. 2 is a diagram illustrating a conventional contention detection operation during a Random Access (RA) procedure. Reference numeral 201 denotes a UE and reference numeral 203 denotes an ENB.
Referring to FIG. 2, the UE 201 selects one of predetermined code sequences called RA Preambles randomly or in a predetermined rule and transmits the selected RA Preamble on an asynchronous RA CHannel (aRACH) in step 211. In step 213, the ENB 203 transmits an RA Preamble response message including an RA Preamble Identifier (id or index) for the RA Preamble, Timing Advance (TA) information by which to adjust UpLink (UL) timing synchronization, Grant information indicating UL resources allocated for transmitting Layer 2/Layer 3 (L2/L3) messages, and a Temporary Radio Network Temporary ID (T-RNTI) being a temporary UE ID. After receiving the RA Preamble response message, the UE 201 checks the RA Preamble id and if the checked RA Preamble id is identical to that of the transmitted RA Preamble, it transmits an L2/L3 message in the uplink resources in step 221.
If a plurality of UEs transmit the same preamble to the ENB 203, contention occurs in the RA procedure. To notify the UE 201 of the successful reception of the RA Preamble, the ENB 203 schedules a Contention Resolution (CR) message including an ID specific to the UE 201 or a random ID received from the UE 201 using the T-RNTI and transmits it to the UE 201 in step 223. For this purpose, the UE 201 includes its unique ID or a smaller random ID in the L2/L3 message of step 221.
Each UE having a T-RNTI can determine from the ID included in the CR message whether it has won or lost the RA contention. If the CR message scheduled using the T-RNTI does not include the unique ID or random ID of the UE 201, the UE 201 re-starts the RA procedure, considering that it has lost the contention.
The use of the CR message clearly indicates to a user as to whether it has won or lost an RA contention. However, since a CR message need to be transmitted on the DownLink (DL) for each T-RNTI at each RA in the conventional procedure, signaling overhead is increased in terms of overall system performance.